… how I learned to stop worrying and love the bomb.
This is the last post in our series of posts on nuclear energy. Here, I shall describe the basic principles behind the design and continued operation of a nuclear reactor. In our last post, we looked at the techniques responsible for making fissile matter release energy as quickly as possible (in a nuclear explosion). Today, we will instead look at techniques to control nuclear fission reactions and usefully harness the resultant energy. Since there are a variety of various nuclear reactor designs and fuels, I shall stick to talking about a reasonably common (although a bit aged) design known as the pressurized-water reactor using U-235 fuel.
A Critical Situation
Nuclear power is most easily derived from fissile fuel – elements that can undergo chain reactions when they fission. We learned in the this post how a chain reaction, when deliberately made uncontrolled, would release in massive amounts of energy. We also learned that such reactions best occur spontaneously if the fuel present exceeds a particular configuration known as the critical mass. For a controlled nuclear reaction, we want to opposite conditions to hold by default. A reactor, when it is not running, should not be critical. When it is functioning, a reactor shouldn’t release all the energy in its fuel at once. Instead, the energy release should be gentle and controlled. To this effect, a number of crucial design decisions emerge. Unlike in a nuclear bomb, the uranium fuel used is not all (or even mostly) U-235. The fuel used is about 2-5% mixture of the fissile U-235 with the rest of it being the non-fissile U-238 isotope. This ensures that fuel isn’t too concentrated and doesn’t become critical easily. A nuclear reactor positions its fuel in a sequence of thin tall rods, unimaginatively called fuel rods. These fuel rods are separated by gaps and are held in position. They are usually held in a fixed pattern in bunches called fuel elements. This geometry makes it even harder for the fuel to reach criticality. What this means is that, when initially assembled, a nuclear reactor does not generate much heat. It simply sits there, gently undergoing radioactive decay normally.
Everything in Moderation…
Well, now we have a reactor that does nothing. How do we get power from it? We need to change the configuration such that the reactor does become critical, but in a controllable way. If the reactor is sub-critical, then the chain reaction can not proceed. If the reactor is super-critical, then the number of fission reactions are increasing, causing an increase in power. When the reactor is just critical, it maintains the number of fission that happen per second, and hence have a constant power output. One way to make a sub-critical reaction critical is to increase the number of neutron/U-235 collisions. Neutrons released during U-235 fission are fast and hence don’t easily continue the chain reaction. If we introduce some water, the fast neutrons can collide with the protons in a water molecule to slow down and shed their energy. This technique of increasing the number of slow neutrons to increase fission rate is known as moderation. The substance used to moderate a nuclear reaction is known as a moderator. In the reactor that we are discussing, water acts as the moderator for the reaction. The resultant neutrons are known as thermal neutrons because their speeds follow the Maxwell-Boltzmann distribution.
Now, we have a reactor that consists of a vessel with reasonably spaced fuel rods sloshing in water. This is not a very safe situation. The slowed down neutrons initiate a lot of fissions and reactor is super critical. Left in this state, this will result in a nuclear power excursion with catastrophic consequences. We need to be able to control the reaction. We also need to be able to safely shut down the reactor when we need to stop it. For this purpose, the nuclear reactor is equipped with a number of control rods. These rods are made of a cadmium-silver-indium or boron alloy that is known for being able to absorb neutrons without undergoing fission itself. The cadmium atoms absorb slow, moderated neutrons to form heavier cadmium isotopes. These isotopes are still not radioactive and simply remain in the control rods. After enough neutron bombardment, the rods eventually become saturated and brittle at which point, they will need to be replaced. These control rods can now be used to control the reaction rate in a nuclear reactor. They are usually partially inserted into the vessel that holds the fuel rods and the moderator. If inserted all the way, they absorb most neutrons that are produced to immediately shutdown the chain fission reaction in the reactor. As the rods are withdrawn, the number of neutrons absorbed decreases and the reaction rate slowly increases. This can be gently varied until the desired power level of the reactor is obtained. In an emergency, safety mechanism can rapidly slam the control rods into the core of the reactor, rapidly stopping the critical chain reaction. This process is usually called SCRAMing or tripping the reactor.
Keepin’ it Cool
Ok! We have a reactor with fuel rods, moderator to slow down and thermalize neutrons and control rods to remove excess neutrons and prevent a power excursion. We still need to harness the power emitted. All the reactor is doing now is emitting lots of heat, making it a deadly area heater. We extract heat from it in the same way we extract heat from burning coal in a coal plant of steam locomotive – we make some steam and use it to drive a power turbine. The substance that is used to draw heat away from a nuclear reactor is known as the coolant. In the reactor we have just been discussing, we already have a convenient coolant available – water! The water that we use to moderate the nuclear fuel also absorbs heat from the reactor. We can pump cool water into the reactor and extract hot water out of it. In certain other reactor designs, the coolant and moderator are different due to various desired properties. In our reactor design, we do have one thing to be worried about – will heating water affect its properties as a moderator? It does, in a possibly helpful way. When water is heated, it expands, lowering its density, and increasing its average molecular velocity. Hence, when neutrons don’t collide with it as often, and when they do, they aren’t slowed down enough. This will, in general, result in fewer fission reactions, causing the reactor to “pull back” and cool down. On the other hand, we don’t want water to become steam. Steam is hard to pump. Steam tends to bubble up and create cavities. These steam pockets cause parts of the reactor to become extremely hot as heat is not conducted away swiftly and efficiently. Nuclear reactors release a tremendous amount of heat which can easily boil water at atmospheric pressures. Hence, in our nuclear reactor, we pressurize the water to keep it liquid and pump the high pressure water through the reactor. The water can now reach much higher temperatures while still remaining liquid. This design is known as the pressurized water reactor (PWR). It is common in naval reactors and commercial reactors. There are other designs that allow water to boil without pressurization called boiling water reactors (BWR). The nuclear reactor at Fukushima is an example of a BWR.
Since we have a reactor coolant that is under extremely high pressure that has been exposed to radiation, it makes sense to leave that water in a closed loop. A standard configuration to extract heat from the pressurized reactor coolant is to pass it through a heat exchanger. Despite the impressive complicated name, this is as simple as passing the metal tubes of hot water through another tank of water. This way, the contaminated, pressurized water stays in the high pressure loop and only conducts heat to water in a separate tank that is not under pressure, boiling it in the process. The boiled water can then be released over turbine vanes, causing them to rotate and generate electricity. The steam can then be condensed and re-circulated. Any excess heat can also be used to provide heating to local houses, swimming pools, hospitals etc.
Throughout this article, we have methodically built up the design of our nuclear power plant based on its power generation requirements. We did not really consider the safety aspects of operating it. Nuclear plants in real life are constructed to be extremely safe with a number of redundant systems using a strategy known as defense-in-depth. Every system has backups and many backups have fallbacks and backups themselves. No single failure should be able to compromise the entire operation. But to understand this better and to trust it, we need to understand the dangers that a nuclear reactor would face and what is being done to protect the facility, its operators and the surrounding commuity. Let’s systematically examine the risks.
Radiation Danger Captain!
Since we are dealing with a radioactive source, an obvious problem is one of radiation exposure. Radiation is emitted during normal operation and during an abnormal power excursion. During a power excursion, the core accidentally becomes super critical and the reactor is in danger of a nuclear explosion. Most reactors guard against this situation by having control rods automatically deploy and by designing the moderator in such a manner as to reduce its effectiveness when it is heated too much. Hence, the danger of an actual nuclear explosion is minuscule. Hence, the radiation we need to worry about is the one emitted during regular operation. Uranium by itself, as examined in this post, is relatively harmless. What is harmful is the fission products (and their products) that have a relatively long half-life. These hang around and decay fast enough to be a problem but slow enough that they don’t disappear within seconds. Hard gamma rays, X-rays and high energy neutrons represent the biggest problems. We can protect the reactor, its operators and the public from this by enclosing the entire reactor in a massive, thick containment shell. The shell is thick and made of radiation absorbent material. Usually this same shell is also made with material that has a high melting point and can withstand earthquakes. A second factor working to mitigate the danger is the fact that the effect of radiation drops off with the square of the distance.from the source. This is because the further away you are, the smaller the fraction of the radiation that affects you as opposed to everyone else at the same distance. This effect doesn’t take into account that the further away you are, the higher the chances that air or other intervening structures such as walls or trees absorb the radiation. Most particulate radiation are quickly absorbed by air and don’t travel very far. It’s the gamma rays that travel relatively uninterrupted and cause a carnage when they encounter DNA or other crucial cells in humans.
Loss of Coolant
We also deal with lots of heat in a nuclear power plant. If things aren’t kept cool, things go downhill in a hurry. If a reactor develops a leak in its cooling loop, the pressurized coolant (water) can rapidly escape. This is an issue in and of itself – exploding water is not an easy thing to deal with. But engineers have had plenty of time to design safe boilers and the same techniques apply here. There are relief valves to ensure that the super heated water and steam do not destroy the core of a reactor. There are techniques of isolating leaking sections to make sure that whatever coolant remains is also not leaked out. There are multiple pumps and cooling circuits to ensure that when one fails, the others take up the slack. In an emergency, the reactor trips and the control rods are rapidly slammed into the core, bringing the chain reaction to a screeching halt. The fission intermediaries are still active and take a while to cool down and completely disintegrate. Hence, shutting down a fission plant is not like turning off a natural gas plant. You need to keep cooling the core down even when it has been shutdown. It’s like turning off an electric burner – it still remains hot for a while. If you notice the construction of most nuclear plants, you will notice that they are situated near bodies of water such as rivers, lakes and seas. This is done deliberately – it ensures that in the case of an emergency where all coolant is lost, the entire core can be flooded with cool water from the nearby body of water to keep it cool and take away excess heat. Sea water is corrosive and hence, care needs to be taken when using it, which is why it is used as a last resort. All this cooling operation also requires power – power that won’t be available when a plant is shutdown due to an emergency. Hence, most power plants come with generators and battery backups, and can pull power back from the power grid to power its cooling systems. Some plants go as far as to use the inertia from spinning generators or wound springs to temporarily power the plant for seconds while extremely critical operations such as scramming the core are performed. In the worst of worst cases, if coolant is lost, the control rods still deploy. The core continues to heat and melt. The heat causes the control rods to melt too. The slush of fuel and control rods now cannot get critical because there is a neutron absorber mixed in with the core. It is still immensely hot and emits lots of harmful radiation but the melted core drops onto the ground and starts dissipating. This is known as a nuclear meltdown. This melted core spreads, increasing the area of exposure of the molten core, causing it to cool down faster. This process can take many decades in a worst-case scenario, but a nuclear explosion does not occur and major disaster can generally be averted. The only major danger during this time is any accidental exposure to lots of water (such as ground water) as this can cause the water to boil violently, which can cause a steam explosion, spreading radioactive waste. Another potential problem is that of chemicals used in the core exploding if they aren’t vented away in time. The Chernobyl nuclear reactor used graphite (a pure form of carbon) as a moderator. When hot carbon is exposed to air, it combusts rapidly, causing a fireball that flings radioactive material into the surrounding environment. Controlling environmental chemicals is a big factor in reactor safety.
Another common scenario that people worry about is externally imposed catastrophes such as terrorism, bombings, computer system failures and the like. The same containment shell that prevents internal explosions from exposing the core can also be made to protect the core from bombs, missiles and crashing aircraft. Computer systems can contain code that is written in a manner that they can be proven unhackable before using it. This is an extremely expensive procedure, but can be done if enough effort is applied. Computer systems are also redundant – central control systems usually are designed to allow for failure. Systems are designed such that each decision is not made by one component but by many separate components. The component should agree during regular operation. During irregular operation, if one of the components produces an invalid or incorrect action, the other components detect this and kick the errant node out of the decision-making procedure. Components also have complex procedures to elect computerized leaders and hold votes to eject errant computer systems out of critical processes.
And now, at the end of this long series on radioactivity and nuclear power, you have a basic understanding of nuclear power, radiation, its dangers and how to harness it – creatively or destructively.